12 Gigabyte cable modem?

We have been paying for a 12 gigabyte cable modem service which isn't cheap by the way for many years now. However, the last year or two this isn't what we have been receiving. When my son and daughter arrived they were dismayed at the poor service and slow computers (relatively speaking). So, my daughter got on the phone and we found out that my son had changed modems without notifying the service which was a part of the problem about 2 years ago. However, then after fixing the problem once again today we were receiving once again only about .5 service instead of the 12 gigabyte service we are paying for.

Now, I know a cable modem is sort of like a freeway, in other words is is not a T1 or T2 line(which is a private line).  HOwever, a cable modem you are sort of on a freeway (so to speak) with everyone else in your area with a cable modem. But, if you are promised 12 gigs and are getting .32 instead of 12 ,   1/3 of 1 is 1/36th of what we should be getting.   We likely deserve a refund for the last couple of years of not getting the 12 gigabyte cable modem line ongoing in regard to actual service that we were promised ongoing.

We should be able to run and download without interruption 12 computers and other stuff at the same time in a speedy and timely way. My daughter is back on the phone again complaining about how this is not what we are paying for. I suppose I could change to a satellite  service away from Comcast if this doesn't get better soon.